Molluscs, zooplanktonic

Zooplankton population samples for isotope analysis were composites of 50-200 individuals. Population samples are less variable in isotope composition than are samples of individuals. Replicate isotope analyses of composite samples of zooplankton or POM collected at different locations within the lake varied by no more than 0.5%. Larger organisms such as molluscs, insects, and fish were analyzed individually. Molluscs were soaked in dilute HCl to remove carbonates and then rinsed copiously with distilled water. Fish muscle was analyzed. Sediment trap material was collected in replicate cylinders (11.4-cm diameter, 76.2-cm length) suspended at 4.5-m depth. All isotope samples were dried at 60 °C before analysis. 

The zooplankton include a number of major taxonomic groups of animals, either for their entire life cycle or for short periods, as in the case of the larval stages of some fishes, polychaetes, crustaceans and molluscs. From among these types of zooplankton by far the most important group are the Crustacea, and of these, the copepods are the most predominant and have thus received most attention (Crisp, 1975). 

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